Hydrocephalus commonly results from an imbalance between CSF production within the brain and re-absorption of CSF into the vascular space. Problems of impaired CSF re-absorption are most common in infants related to perinatal intracranial bleeding and in elderly individuals with slowly progressive impairment of CSF re-absorption (“normal pressure hydrocephalus”). Another cause can be impairment of flow of CSF through the brain's ventricular system (obstructive hydrocephalus). It is estimated that almost 200,000 individuals are diagnosed with hydrocephalus each year in the US. An additional group of patients develop obstructive hydrocephalus due to tumors or other pathologies affecting CSF flow in the brain.
The majority of shunts drain CSF by way of a catheter extending from the lateral cerebral ventricle through brain parenchyma, and a burr hole in the skull, to the sub-galeal space beneath the scalp. A pressure-controlled flow valve is connected to this catheter, and the outlet is connected in turn to a distal catheter that drains into a space capable of re-absorbing CSF, such as the peritoneal space in the abdomen. Placement of a shunt is accomplished by way of incisions over the burr-hole site on the scalp and the distal catheter placement site (abdomen, chest or neck). After connecting the ventricular and distal catheters to the valve, the incisions are sutured, leaving the entire system beneath the skin surface. Placement of a shunt is always carried out under controlled sterile conditions, and any repairs or changes of components require additional procedures in the operating room.
Shunt malfunctions can be considered in terms of providing inadequate or excessive CSF drainage. Browd S R, Gottfried O N, Ragel B T, Kestle J R., “Failure of cerebrospinal fluid shunts: part II: overdrainage, loculation, and abdominal complications”, Pediatr Neurol. 2006; 34:171-6. The most common malfunctions result in complete or intermittent blockage of CSF flow. Clinical techniques for determining whether a shunt is blocked or malfunctioning are the subject of considerable discussion (Browd et al., 2006). This is primarily because no external means of gauging shunt functioning has been available. The technique of choice involves placement of a needle through the scalp into a bulb-shaped reservoir that is typically built into the valve housing upstream of the valve mechanism (“shunt tap” procedure). However, this approach is not generally reliable and carries a risk of shunt infection, which are virtually impossible to eradicate without replacing the shunt surgically. Rocque B G, Lapsiwala S, Iskandar B J, “Ventricular shunt tap as a predictor of proximal shunt malfunction in children: a prospective study”, J Neurosurg Pediatr. 2006; 1:439-43. The only other alternative, exposing a valve surgically to ascertain its function, would require a trip to the operating room, typically under general anesthesia.
One of the main ways that shunt malfunctions are diagnosed is the development of recurrent symptoms of hydrocephalus. Given that the earliest signs of shunt malfunction are often severe headaches, somnolence or cognitive impairment, it can be appreciated that simple ways to establish whether a shunt is patent and to determine the patient's intracranial pressure would be of great value.
Cerebrospinal fluid (CSF) shunts have revolutionized the treatment and outcomes for hydrocephalus since their invention almost 60 years ago. Drake J M, Kestle J R, Tuli S., “CSF shunts 50 years on—past, present and future” Childs Nerv Syst. 2000; 16:800-4. While these relatively simple devices have been effective in treating hydrocephalus, they are prone to failure, infection and other complications, leading to a recent panel recommendation for improved shunt technologies (NIH Proceedings 2005) and a recent NIH Program Announcement promoting grant applications to develop promising new technologies (NIH Program Announcement PA-09-205).
A particular problem lies in the fact that once implanted, the only indication of implant failure is the recurrence of clinical symptoms. Implanted shunt valves to drain CSF constitute a key element in the therapy of various forms of hydrocephalus. However, the implanted devices are prone to blockage. Currently, the only indication of shunt malfunction is the recurrence of clinical symptoms.